Neutrinos – the next big small thing

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"UNEASY lies the head that wears a crown," wrote Shakespeare. The same could be said today of the standard model of particle physics, our most successful description of the building blocks of matter and their interactions. The recent discovery of a particle that looks very much like the Higgs boson stands as the theory's crowning achievement, validating a prediction made nearly four decades ago and filling the model's last major gap. Yet we are as eager as ever to knock it from its throne, to discover the new physics that must surely supersede it. "The standard model is particle physics," says Nobel prizewinning physicist Jack Steinberger. "But there are many unanswered questions that are extremely elusive at the moment."

Those questions include the nature of dark matter - the mysterious, invisible material thought to make up more than 80 per cent of the mass of the universe. Then there is dark energy, the stuff reckoned to be causing the universe's expansion to accelerate. In what must rank as our worst prediction, particle physics overestimates dark energy's magnitude by a factor of 10120. The standard model also cannot explain how matter survived the big bang, or how gravity fits into the picture. It is riddled with so-called "free parameters", troublingly arbitrary numbers that have to be fed into the theory by hand, for example to set the strength of the interactions it describes.

Researchers had hoped that the Higgs would lead to the new physics that is needed to explain away these difficulties. But with the Higgs behaving largely as expected so far, the real key to the kingdom beyond the standard model may lie with a different sort of particle: neutrinos.

Neutrinos hit the headlines in September last year when the OPERA experiment under Gran Sasso mountain in Italy clocked them apparently travelling faster than the speed of light, an activity forbidden by Einstein's special theory of relativity. Six months later, the finding was traced to a glitch in the experiment. Even so, there is plenty more to say and learn about these beguiling particles.

Ghostly, mysterious and antisocial - they rarely deign to interact with the world of common matter around them - much of what is known about neutrinos lies outside the standard model. The three neutrinos we know about fit neatly enough. They pair with the electron and its two heavier cousins, the muon and the tau. A trio of antineutrinos also exists, which pair with the positively charged antiparticles of the electron, muon and tau to complete the extended lepton family (see chart). But at the outset, the standard model wrongly assumed neutrinos have no mass, and even now it cannot specify the masses they do have. It did not foresee their ability to shape-shift from one type into another, nor the fact that there might be more than three of them.

Many new theories hope to fill in those gaps, including grand unified theories, supersymmetry and string theory. One of them might gain traction by explaining why neutrinos are so very weird. Neutrinos themselves might in turn tell us which theory is on the right track.

Despite their aloof nature, neutrinos have a long history as problem-solving particles. Physicist Wolfgang Pauli conceived of them in 1930 in order to conserve energy and momentum in radioactive beta decays. More recently, neutrinos have moved to the forefront of our efforts to explain how matter came to dominate antimatter in our universe.